Objective:

The release of light non-aqueous phase liquids (LNAPLs) into the subsurface is a well-documented environmental concern. Past practices have led to extensive LNAPL groundwater contamination, mostly in the form of petroleum products. Potential for further contamination is also a concern when considering the presence of LNAPL sources in the subsurface (e.g. underground storage tanks and oil/gas pipelines). Surfactant enhanced aquifer remediation, has emerged as one of the most technically effective and cost-competitive technologies for remediation of LNAPL contamination. This technology has applications to filling stations, refineries, and even active well sites. While advances in surfactant chemistry have dramatically improved LNAPL removal efficiencies, the key to further improvements in the economic competitiveness of surfactant-based technologies is to reduce the mass of surfactant needed to recover the free-phase LNAPL. Previous work by the investigators found that some biologically produced surface active agents, biosurfactants, can remove a large percentage of residual hydrocarbon from sand-packed columns at biosurfactant concentrations 10 to 100-fold lower than typically used for surfactant-enhanced LNAPL mobilization. The objective of this research is to assess the relative technical and economic efficiency of synthetic surfactants versus biosurfactants for fear of LNAPL contamination.

Surfactant flushing of contaminated subsurface environments has emerged as one of the most technically effective and cost-competitive technologies for remediation of LNAPL contamination. Surfactants (surface active agents), commonly known as soaps or detergents, are amphiphilic molecules that have both water-like and oil-like regions to their molecule. Because they are amphiphilic, surfactants are surface active molecules, meaning that when they are placed in water-oil or water-air systems, they accumulate at the interface with their water-like region in the polar water phase and their oil-like region in the nonpolar oil or less polar air phase. In this way, both regions of the molecule are in a preferred phase and the free energy of the system is minimized.

When the aqueous surfactant concentration exceeds a certain level, surfactant molecules self-aggregate into spherical structures known as micelles, which contain fifty or more surfactant molecules. Micelles form when the surfactant concentration exceeds the critical micelle concentration (CMC). Micelle formation is unique to surfactant molecules, and differentiates them from alcohols, which do not form such aggregates. Surfactant micelles increase the aqueous concentration of low-solubility organic compounds by providing a hydrophobic region into which organic compounds can partition. The micelle concentration increases with increasing surfactant concentrations above the CMC. The apparent solubility of the contaminant increases correspondingly. Surfactant concentrations well above the CMC (e.g., 10 to 20 times the CMC or more) are used to maximize contaminant solubility and extraction efficiency. The use of a single surfactant to enhance solubility is called solubilization. While this is a fairly straightforward approach to enhancing NAPL dissolution, it may not be the most efficient approach. By using a mixture of surfactants, the water-NAPL interfacial tension is dramatically reduced which further improves the solubility of NAPL. Intentionally lowering the water-LNAPL interfacial tension to displace entrapped NAPL is a process called mobilization.

Biosurfactant production has traditionally been viewed as a mechanism to enhance hydrocarbon biodegradation by increasing the apparent aqueous solubility of the hydrocarbon. However, there are several biosurfactants that generate low interfacial tensions between the hydrocarbon and the aqueous phases required to mobilize residual hydrocarbon. In particular, the lipopeptide biosurfactant produced by Bacillus sp. and the rhamnolipid produced by various Pseudomonas species reduce the interfacial tension between certain hydrocarbon and aqueous phases to very low levels (<0.01 mN/m). In addition, the critical micelle concentrations are low 20-50 mg/l, indicating that the biosurfactants are effective even at very low concentrations. Thus, surfactant enhanced subsurface technology is one of several innovative technologies that is being widely evaluated for remediation of subsurface NAPL spills. Recent advances have helped to make this technology economically viable even when using higher concentrations (1,000 to 40,000 mg/L) of synthetic surfactants. However, recent work with biosurfactants shows that these materials can produce similar removal efficiencies with much lower surfactant concentrations (say 1 to 2 orders of magnitude lower). Thus, the economics of this technology may be even more favorable using biosurfactants, while also improving the environmental friendliness of implementing this technology (biosurfactants versus synthetic surfactants).

The objective of this research is to assess the relative technical and economic efficiency of synthetic surfactants versus biosurfactants used to recover free-phase LNAPL. Laboratory soil column tests were conducted to identify optimal solutions of synthetic surfactant and biosurfactant solutions for mobilizing LNAPL. Until now, the interfacial activity and efficacy of recovering residual hydrocarbon has only been studied with individual biosurfactant compounds. Our work evaluated not only the efficacy of individual biosurfactants, but also whether mixtures of different lipopeptides and/or rhamnolipids have enhanced interfacial activities.

Progress and Final Reports:

Main Center Abstract and Reports:

Subprojects under this Center:(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).R830633C001 Development of an Environmentally Friendly and Economical Process for Plugging Abandoned Wells (Phase II)R830633C002 A Continuation of Remediation of Brine Spills with HayR830633C003 Effective Stormwater and Sediment Control During Pipeline Construction Using a New Filter Fence ConceptR830633C004 Evaluation of Sub-micellar Synthetic Surfactants versus Biosurfactants for Enhanced LNAPL RecoveryR830633C005 Utilization of the Carbon and Hydrogen Isotopic Composition of Individual Compounds in Refined Hydrocarbon Products To Monitor Their Fate in the EnvironmentR830633C006 Evaluation of Commercial, Microbial-Based Products to Treat Paraffin Deposition in Tank Bottoms and Oil Production EquipmentR830633C007 Identifying the Signature of the Natural Attenuation in the Microbial Ecology of Hydrocarbon Contaminated Groundwater Using Molecular Methods and “Bug Traps”R830633C008 Using Plants to Remediate Petroleum-Contaminated Soil: Project ContinuationR830633C009 Use of Earthworms to Accelerate the Restoration of Oil and Brine Impacted Sites

The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.